For general information, reference is made to my earlier U.S. Applications "ELECTRODIALYSIS APPARATUS," Ser. No. 08/784,050, filed Jan. 17, 1997 now U.S. Pat. No. 5,972,191 and "APPARATUS AND PROCESS FOR ELECTRODIALYSIS OF SALTS," Ser. No. 08/787,899 filed Jan. 23, 1997.
The invention relates to a use or addition of certain organic acids, particularly polyaminoacetic acids, in order to enhance the retention in a solution of divalent cations in the pH range of 2 or higher. The stoichiometric complexes, called "chelates," formed with the multivalent cations, are effectively retained in a solution by an ion exchange membrane so that they are not transported out of a given process loop. The primary benefit of this procedure is that the divalent metals are kept in the solution within a given loop of the electrodialysis cell and, more particularly, in the vicinity of bipolar membranes used in cells, as described in my earlier applications. This retention, in turn, has a remarkably beneficial effect on the cell's operation in terms of improving the process' reliability, reducing power consumption and avoiding heating/melting problems.
Nanofiltration, chelating resin ion exchange, or other pre-treatments may be used in conjunction with the electrodialysis step to reduce the divalent and multivalent metals content in a salt feed stream or a base product stream. Within the electrodialysis cell itself, monovalent selective cation membranes may be used to further reduce the amount of divalent metals transported out of the feed loop and thereby reduce the amount of a chelating agent needed to retain the divalent metals in the transported product (usually higher pH) solution.
The invention enables essentially the same process improvement to be used in many different systems, without requiring an extensive modification thereof. More particularly, the inventive process may be used in these and a number of other applications such as (a) the production of acids, organic or inorganic, in conjunction with a weak base such as ammonia. In this context, the invention is particularly well suited to a recovery of lactic acid from a fermentation derived ammonium lactate, especially a recovery in a two compartment cell, (b) recovery of sulfur dioxide from waste gases using a two compartment cation or anion cell, and (c) purification of ammonium salts for use in a fluid catalytic cracking ("FCC") catalyst production using a two compartment anion cell, and (d) production of acids and bases in a three compartment cell.
Salts are byproducts or intermediate products of a number of chemical processes. Regenerable flue gas desulfurization processes use a sodium alkali to absorb the SO.sub.2, thus resulting in a soluble bisulfite salt, NaHSO.sub.3. Fermentation processes for organic acids (such as acetic and lactic acids) go through the intermediate production of salts, such as ammonium acetate or lactate. For example, the manufacturing of rayon/regenerated cellulose results in a generation of significant quantities of byproduct sodium sulfate.
Electrodialysis ("ED") may be used to convert these and other soluble salts directly into their acid and base components. For example, in the case of organic salts, such a procedure will enable a direct recovery of the organic acid in a relatively pure form, while the co-product base (ammonia for example) may be recovered for reuse in the fermentation process in order to make pH adjustment. Thus, there is an economical and environmentally superior option for producing organic acids. In other instances, such as with sodium bisulfite or sodium sulfate, electrodialysis offers an environmentally superior route for recovering and/or recycling the acid, base components.
Electrodialysis uses direct current as a means for causing a movement of ions in a solution. Electrodialysis processes are carried out in a stack arrangement comprising a plurality of flat sheet ion exchange membranes. To produce acids and bases from their salts, the process unit requires a means for splitting water. Two useful means for splitting water into hydrogen (H.sup.+) and hydroxyl (OH.sup.-) ions are:
(i) A bipolar membrane or a bipolar module including a combination of cation and anion membranes functioning as a bipolar membrane. Suitable bipolar membranes are available from Aqualytics, a division of Graver Water and Tokuyama Corporation. PA1 (ii) An electrode set comprising an anode and a cathode. The electrodes, (particularly the anodes), are suitably coated for chemical stability, for minimizing power consumption, and for the formation of byproducts other than hydrogen (at cathode) and oxygen (at the anode). Suitable electrodes are available from Eltech Corporation, Electrode Products Inc., and others. One can also use a hydrogen depolarized anode to generate the H.sup.+ ions in the aqueous solution and next to the anode.
When using a stack of bipolar membranes, the stack contains electrodes (anode and cathode) at either end of a series of membranes and gaskets which are open in their central area in order to form a multiplicity of compartments separated by the membranes. Usually, a separate rinse solution is supplied to end compartments which contain the electrodes with special membranes placed next to the electrodes to prevent a mixing of the process streams with the electrode rinse streams.
The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. The repeating unit is called a "unit cell" or simply a "cell". A solution is usually supplied to the compartments either by internal manifolds formed as part of the gaskets and membranes or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another stack in order to optimize process efficiency. The change in the composition of a stream after one pass through the stack may be relatively small.
The solutions can be recycled by being pumped to and from recycle tanks. An addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
A known treatment of aqueous salt streams by electrodialysis (or electrolysis) forms an acid and a base from the salt. In order for a bipolar membrane, to function as a water splitter, the component ion exchange layers must be arranged so that the anion selective layer of each membrane is closer than the cation selective layer to the anode. A direct current is passed through the membranes in this configuration to cause water splitting with OH.sup.- ions being produced on the anode side and a corresponding number of H.sup.+ ions being produced on the cathode side of the membranes. The dissociated salt anions move toward the anode and the dissociated salt cations move toward the cathode.
The electrolysis process works in a similar manner, with the water splitting occurring at the two electrodes. When a direct current is passed, water molecules are converted to oxygen gas at the anode along with an introduction of H.sup.+ ions into the aqueous solution. At the cathode, the water molecules are converted to hydrogen gas along with the introduction of OH.sup.- ions into the aqueous solution. In the hydrogen depolarized anode based electrolysis unit, OH.sup.- ions are released into the aqueous solution next to the cathode, while the released hydrogen gas is forwarded to the catalytic hydrogen depolarized anode for H.sup.+ ion generation.
Electrodialysis equipment for acid and base production has three compartment cells comprising bipolar, cation and anion membranes, two compartment cells containing bipolar and cation or anion membranes, a multichamber two compartment electrodialysis cells comprising bipolar and two or more cation membranes (or two or more an ion membranes). A number of processes uses such equipment.